The development of efficient, low noise power converters is a continuing goal in the field of power electronics. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunications and computer systems wherein high voltages are converted down to lower voltages needed to operate the systems. Power converters generally suffer from problems such as switching losses, switching noise and common-mode power transformer noise. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment.
Current power converter designs often implement one of two full bridge topologies: the conventional full bridge and the phase-shifted full bridge. Both topologies include four switches, typically power metal-oxide semiconductor field-effect transistors (MOSFETs), operated in alternating pairs, an input/output isolation and step-up/step-down transformer, an output rectifier, and an output filter. A feedback regulator or controller is included to control the switches.
The conventional full bridge operates generally as follows. The switches are arranged in two diagonal pairs that are alternately turned on for a fraction of a switching period to apply opposite polarities of the input DC voltage across the primary of the transformer. Thus the switches operate to convert the input DC voltage into an AC voltage required to properly operate the transformer. Between conduction intervals of the diagonal pairs, all the switches are turned off for a fraction of the switching period. Ideally, this should force a voltage across the primary of the transformer to zero. A rectified voltage of the transformer should, therefore, ideally be a square wave with an average value proportional to a duty ratio of the diagonal pairs.
The output filter decomposes the rectified voltage into AC and DC components. The DC component is an output voltage of the power converter. The output voltage is generally fixed. The feedback regulator, therefore, monitors the output voltage and adjusts the duty ratio of the diagonal switches to maintain the output voltage at a constant level as the input DC voltage and load current changes.
In practice, the rectified voltage is not a perfect square wave, however, because turning off all the switches induces a ring between a leakage inductance of the transformer and a parasitic capacitance of the switches. The ringing dissipates energy, thereby reducing the efficiency of the power converter. The ringing also gives rise to significant conducted and radiated electromagnetic interference.
The phase-shifted full bridge power converter was developed to alleviate the switching loss and switching noise problems of the conventional full bridge. The construction of the phase-shifted full bridge is essentially identical to that of the conventional full bridge. Its advantages result, however, from the operation of the switches to produce a zero voltage across the transformer. Instead of turning off both switches of each diagonal pair to begin the zero voltage period, the phase-shifted full bridge turns off only one switch of the pair. A switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate at zero voltage through the two switches. The two switches clamp the voltage across the transformer at zero, thereby eliminating the ringing behavior suffered by the conventional bridge when the switches are off. By clamping both ends of the transformer to one rail, then to the other rail, however, the phase-shifted full bridge induces transients on an intrinsic primary to secondary capacitance of the transformer. As a capacitor potential is alternately charged from rail to rail, common-mode noise is thereby generated.
Accordingly, what is needed in the art is a system and method is for clamping the voltage across the transformer to zero (to reduce switching losses and switching noise) without inducing noise on the intrinsic capacitance of the transformer, thereby generating less common-mode noise in full bridge power converters of the type described above.